U.S. Pat. No. 5,915,679 (KOHLERT) describes a workpiece holding element with a support bolt slideably movable and guided in a bore of a housing. Once the support bolt has reached its clamping or rather support position in response to an air stream, a locking piston is pressed, for example by a hydraulic force, against the support bolt in a direction perpendicularly to the longitudinal bolt axis to hold the support bolt in place.
When clamping unstable work pieces on a machine tool or on pallets, it is necessary to perform the clamping in such a way that the work pieces are not deformed and that a located workpiece is not displaced out of its precise, located position. Thus, excessive forces may not be applied to the work pieces. However, it is nevertheless necessary that the work pieces are in a stable position for the machining operation. Especially for unstable work pieces a repeatable, fine adjustment of the force applied to the workpiece to keep it in place is required and such requirement can be satisfied only by special support elements. Normally, more than three support points are required. The support elements must be so constructed that the support bolt can easily be positioned to contact the workpiece and then to lock the support bolt in the workpiece contacting position without deforming the and without displacing the workpiece.
Other conventional workpiece holding elements are equipped with screws, springs or wedges for adjusting and holding the support bolt. These elements are mechanically operated, whereby so-called stretch belts may be used. However, hydraulically operated support elements are also known. Generally, the locking takes place either by maintaining the pressure that moves the support bolt or by a laterally extending force applied to the support bolt. In the above mentioned U.S. Patent to Kohlert the support bolt is mounted in a housing and pneumatically movable into a workpiece contact by air that is introduced into the housing bore that guides the support bolt. The air that moves the support bolt is passing through special guide channels and develops a determinable or controllable contact force between the support bolt and the work piece. The locking of the support bolt is accomplished by a hydraulically operated locking piston extending perpendicularly to the length of the support bolt and pressing the support bolt against the wall of the housing bore.
The hydraulic forces effective through the locking piston on the support bolt during a machining operation assure an absolutely safe holding or locking of the support bolt. This feature means that the frictional forces which are effective on the support bolt by the pressure exerted by the piston must be larger than the reaction forces from the workpiece.
The supporting element of my earlier invention performs its function well. However, there is room for improvement, especially with regard to simplifying the construction and avoiding the need for both a hydraulic source of pressure and a pneumatic source of pressure.